The development of new and improved varistors for use in electrical surge arresters is a continuing concern in view of the ever increasing demand for electricity and electrically powered devices. Varistors are electrical resistors exhibiting a strongly, non-linear relationship between the applied voltage and the resulting current flow. Because of the varistor's non-linear behavior, when a line voltage exceeds the breakdown voltage of this device, the surge is carried away through the varistor and the circuit is thereby protected.
Presently, there exists a variety of varistors, including metal oxide varistors such as zinc oxide varistors and non-metal oxide varistors such as silicon carbide. The metal zinc oxide varistors are ceramics that have highly nonlinear electrical conduction characteristics which make them especially suitable for use as surge arresters or voltage limiters in electrical systems, as opposed to the non-metal oxide varistors which utilize series spark gap devices.
Zinc oxide varistors in surge arresters are subject to low current, long duration impulses, especially when switching conditions prevail in the circuit. The varistors in the arrester must be able to withstand these high energy impulses. Many investigations (U.S. Pat. Nos. 3,760,318; 3,857,174; 3,872,582; 3,903,494; 3,905,006; 3,938,069; 4,031,498; 4,317,101, 4,319,215; 4,326,187; 4,420,737; 4,450,426; 4,460,623; 4,474,718, 4,495,482; 4,692,735; 4,700,169; 4,719,064; 4,724,416; 4,730,179; 4,855,708) in the past have concentrated on improved high current withstand ability, as well as improved varistor stability with respect to high current, short-duration surges and/or operation under steady-state load conditions for long periods of time by coating the varistors with various formulations. It should be kept in mind that the varistors must be coated with an insulating material in order to prevent flashover during these types of electrical conditions on the utility line. U.S. Pat. No. 4,450,426 also addresses an improved low current long duration response for the varistors. However, the coatings in these investigations were applied onto calcined or fully fired devices or with fully non-aqueous based solvent systems. It would be advantageous to arrive at a coating system for application onto an unfired metal oxide component, thereby allowing the coated component to be cofired. It would also be advantageous to develop a carrier system which is aqueous based for the application onto the unfired component thus reducing environmental concerns with the application process. Lastly, once fired, the coated component would provide an improvement in the energy handling response of the resultant metal oxide varistor as well as maintain acceptable nonlinearity and high current, short-duration impulse characteristics for high voltage applications.